Why does the second alert say undefined?

by questwork

HTML

<p><b>Question:</b></p>
Why does the second alert say undefined?
<p><b>題目:</b></p>
为什麽第二次的提示为"undefined"?

<p><a id="clickme">click me 按我!</a></p>

JavaScript

var a = document.getElementById('clickme');
a.onclick = function() {
  alert(this.innerHTML)
  setTimeout( function() {
      alert( this.innerHTML );
  }, 1000);
};